This invention relates to longitudinal mode laser diodes and, more particularly, to laser diodes employing gain-stabilizing feedback by locating a first or second-order grating of suitable pitch externally to the gain region of the solid state structure, more commonly known as DBR lasers.
In a co-pending application entitled xe2x80x9cIncreasing the Yield of Precise Wavelength Lasersxe2x80x9d, assigned to the assignee of the present application, Ser. No. 10/227,033 filed Aug. 23, 2002, a laser device is disclosed in which a short, second-order grating was etched into a semiconductor lying in an unpumped region beyond the end of the gain stripe. Because the short grating was located beyond the end of the gain stripe it was not necessary to cover the grating with gain-providing epitaxial layers which require a contaminant-free surface. The layer in which the grating was etched was located at a distance sufficiently remote from the high intensity optical field of the waveguide to provide just enough feedback to reduce the gain at an unwanted wavelength (xcex2) and yet was sufficient to support oscillation at a desired wavelength (xcex1) without incurring excessive surface diffraction loss. Since the short grating was one-tenth the length of Fabry-Perot cavity, the diffraction coefficient, xcex1, was required to be ten times larger than when a full-length DFB grating is employed, e.g., xcex1≈1.0 cmxe2x88x921. This held the fraction of power lost to surface diffraction radiation to about 1%, which is sufficient to provide stabilizing feedback without sapping too much energy from the longitudinal beam.
As described in the aforementioned application, the surface grating was produced after the wafer, containing most of the many conventional layers, had been fabricated. The wafer was coated with a first, fairly thick photoresist and exposed to light through a mask to define a window area, preferably at the end of the wafer where the grating to be formed would have the least possibility of influencing the optical gain. Development of the photoresist removed it from the window area, but left the remainder of the wafer covered. An etchant applied to the wafer to removed layers above the cladding layer at the defined window. A second, thinner photoresist was then applied and exposed to a holographic pattern of interfering laser beams to form xc2xc micron fine periodic grating pattern. The photoresist was developed and gas plasma etching was employed to transfer the grating pattern into the cladding layer. It was thought that the rest of the wafer which remained covered with the thick photoresist would be protected against the plasma etching. After the grating was formed in the cladding layer, a layer of SiO2 was applied to the entire wafer and selectively removed to permit metal deposition of electrical contacts that would define the gain stripe area.
Unfortunately, it was found that the use of the two-photoresist process did not sufficiently confine the grating to the region at the end of the mesa, the use of a thin photoresist apparently not being completely benign with respect to the underlying thick photoresist. The grating pattern which was desired only at the end of the wafer also tended to be produced over the entire surface area leading to unwanted feedback effects.
A DBR grating may be created in the cladding layer of a wafer by defining a window area, advantageously at the end of the wafer such that the DBR grating created in the window area would have minimal effect on optical gain. The grating window area is preferably defined at the end of the wafer after most of the wafer""s layers have been produced by the usual MOCVD process. All layers above the cladding layer are then removed in the window area. Alternatively, the grating area could be defined by halting the MOCVD process after only the cladding layer has been laid down and by overgrowth of the remaining layers, although overgrowth of gain layers upon an etched layer is somewhat difficult in a production environment. In either case, a protective coating, advantageously of SiO2, Si3N4, or a metal selectively is then applied to the wafer and selectively removed from the window area. A thin photo resist is applied to the entire wafer which is then exposed to interfering laser beams. While the grating pattern is created throughout the photoresist, the protective coating underneath the photoresist prevents the subsequent etching that transfers the pattern into the window area at the end of the wafer from being etched into the remainder of the wafer, without the need for any particular effort to confine either the photoresist or the interfering beams solely to the window area. The protective layer is then removed and other layers may be laid down in the usual manner.